1. Field of the Invention
This invention relates to saw sharpeners, and in particular, to such sharpeners for sharpening teeth of a band or circular saw using an automatically controlled grinding wheel.
2. Related Art
Both band and circular saws are used in lumber mills to cut logs into dimensional lumber. The band saws are typically made in left and right-hand versions so that the return runs or loops of the bands are spaced away from the log being cut and the cutting runs are spaced closely and in parallel to cut a slice of wood having a predetermined thickness. Other sets of band saws cut other portions of the log. Circular saws are typically used to cut the wood after it has been cut into boards. The preferred embodiment of the present invention is designed to sharpen both circular saws and band saws. The following discussion and disclosure are directed specifically to band saws. However, various aspects of the invention are also applicable to circular saws.
The bands are typically 18 to 60 feet long in a continuous loop and have a width of 4 inches to 16 inches. A typical sharpener, such as a Motor Driven No. 4 Automatic Precision Band Saw Sharpener made by Armstrong Manufacturing Co. of Portland, Oreg., sharpens either a left or right-hand band saw. The band saw passes through a sharpening zone or station on the front of the sharpener and extends around behind the sharpener.
The band saw is supported at spaced locations along its length away from the sharpener on post brackets. The saw is moved around in the loop configuration by an indexing mechanism in the sharpener and in a back feed unit that drives the return loop behind the sharpener.
During loading, these units require the return run of the band saw to be lifted over the entire sharpener to get it behind the sharpener. Further, it is preferable to have both left and right hand sharpeners for sharpening left and right hand band saws. One type of sharpener can be used for both left and right-hand saws, however, if the operator is willing to operate the sharpener while standing inside the loop of the band.
The conventional sharpeners also have a grinding wheel that is moved in the plane of the grinding wheel relative to a band saw being sharpened as the band saw is moved through the sharpening zone. The grinding wheel moves according to rotation of a cam that is coordinated with movement of the band saw. The amount of grinding is controlled by manually operated cranks, and consistency of the sharpening depends on operation of the control crank and the shape of the grinding wheel when it is dressed. The resulting sharpness of the band saw is also dependent upon the coordination of movement of the band saw and grinding wheel.
Hot spots on the band saw result from excessive grinding. Such hot spots result when too much grinding occurs at a single location, and results from misadjustment of the sharpener and vibration of the band saw by the grinding wheel. These hot spots are weak and can result during use in fractures or cracks, typically in the gullet between saw teeth. These cracks are due to the band bending as it travels around band wheels, the tension placed on the band, and the cutting loads imposed by the tooth as it shears wood fiber.
In an attempt to prevent the occurrence of hot spots a stream of coolant has been directed into the sharpening zone. However, the high rate of rotation of the grinding wheel, typically in the range of 1800 to 5000 rpm, causes a layer of air next to the wheel to move with the wheel. This layer of air prevents the coolant from getting to the wheel/band saw contact area, and as a result has been found to be relatively ineffective.
As has been mentioned, one of the reasons for the development of hot spots on band saws is the vibration that occurs during contact of the grinding wheel with the band saw. The band saw is clamped below the level of the teeth so that the teeth are fully exposed as they travel past the grinding wheel. Since the grinding wheel is at a fixed location in the general loop of the band saw, the tooth cannot be clamped and still have the opening or gullet between the teeth exposed during sharpening.